Moistureproof microwave antenna

ABSTRACT

A moisture proof hollow electrical component such as a microwave antenna the interior of which is filled with a closed-cell, foamin-place, low-density retarded polyurethane plastic substance having a specific dielectric constant approximately equal to that of the air which it replaces, thereby preserving the original electrical impedance characteristics and hence maintaining the operating efficiency of the antenna.

[ 51 Feb.6, 1973 United States Patent [191 Mackie MOISTUREPROOFMICROWAVE ANTENNA 7/1963 Raabe....... 3,239,838 3/1966 Kelleher......,3,273,158 9/1966 Fouts et al.

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polyurethane plastic substance having a specific [56] References cueddielectric constant approximately equal to that of the UNITED STATESPATENTS air which it replaces, thereby preserving the originalelectrical impedance characteristics andhence maintaining the operatingefficiency of the antenna.

2,948,691 8/1960 Windemuth et al. ............260/2 5 BB 3,054,7559/1962 Windemuth et al. ............260/2.5 AP 3,472,800 l0/l969 Kurylaet al. .............260/2 5 AR UX 1 Claim, 3 Drawing FiguresPAIENTEDFEBQSIQB V 3.715 756 SHEETlUF 2 INVENTOR.

WILLlAM L. MACKIE Arromv'n PATENTIEDFEH 81973 INVENTOR WILLIAM L. MACKIEA TTORNEY MOISTUREPROOF MICROWAVE ANTENNA CROSS REFERENCE TO RELATEDAPPLICATION The present application is a continuation-in-part ofapplication Ser. No. 767,884 filed Aug. 27, 1968 and now abandoned.

STATEMENT OF GOVERNMENT INTEREST The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

BACKGROUND OF THE INVENTION Many high-frequency antenna arrays areformed as a complex network of hollow radiating elements of elongatedconfiguration and restricted diameter arranged symmetrically about acentral point at which a common electrical feed member is located. Inmany cases, this antenna feed member is also of hollow configuration,and is coupled tothe transmission line which leads to the antenna bymeans of a so-called Balun connector. The antenna may or may not includea reflector to control the direction in which the emitted energy ispropagated.

Assemblies of the above type are subject to malfunction in the presenceof moisture. This is primarily due to the fact that the structurebreathes during changes in atmospheric pressure, and water vapor isconsequently drawn into the hollow elements of the unit. This vapor thencondenses and adversely affects the electrical characteristics of theantenna. Attempts have been made to seal the joints with variouscompounds, but this has not proven satisfactory when the customaryplastic-to-metal interfaces are present.

Invasion of moisture into microwave antennas of the nature described maybe prevented by originally designing the structure to incorporate asatisfactory filler material which is embodied into the unit duringmanufacture. This material is of course chosen to be compatible with theparticular electrical and mechanical characteristics of the particularassembly into which it is introduced.

However, when it is desired to protect an existing structure from thepresence of interior moisture, the problem is a serious one. Theexpedient of pressuring the antenna with an inert gas (such as argon ornitrogen) was attempted, but leakage could not be prevented and thestructure retained the gas only for a short period of time. It was thensuggested that the cavities be filled with a high-density foam, but thisresulted in an impedance mis-match in the antenna and seriously degradedits performance. This could have been avoided by employing acompensating transformer, but such extra component not only adds to thecost of the apparatus but introduces an additional source of potentialmalfunction.

SUMMARY OF THE INVENTION In accordance with a feature of the presentinvention, the basic concept of filling the hollow portions of anexisting microwave antenna with a moisture-excluding substance isretained, but the procedure is modified to take into account thenecessity of maintaining the electrical efficiency of the assemblyfollowing such modification without adding separate components such asthe transformer mentioned above. In order to achieve optimum results,three factors have been given consideration. There are (l) the choice ofa filler material such as polyurethane plastic of the rigid,closed-cell, freon-blown,.foam-in-place type having a specificdielectric constant chosen in accordance with the antennas electricalcharacteristics, (2) the addition of a retarding agent to the mixtureprior to insertion into the antenna cavities, so that the compositionmay be injected as a liquid capable of flowing over the internalsurfaces to wet all areas of the latter before hardening of the materialoccurs, and (3) imparting to the plastic material a high expansion rateso that each hollow antenna region is certain to be completely filled bythe foam after expansion of the latter has taken place.

One object of the present invention, therefore, is to provide amoistureproof and electrically insulated microwave antenna, as well asthe connection between such antenna and the transmission line leadingthereto.

Another object of the invention is to provide a highfrequency,multipolarizedantenna which is highly resistant to the entry of moistureunder adverse environmental conditions, such for example as temperaturesin excess of 200 F where certain thermoplastic substances, such aspolystyrene, cannot be employed.

A further object of the invention is to provide a cubic antenna array inwhich each radiating element is completely filled with a moistureproofsubstance, the electrical characteristics of which are chosen to matchthose of the antenna, whereby a high level of operating efficiency ismaintained.

Other objects, advantages, and novel features of the invention willbecome apparent. from the following detailed description of theinvention when considered in conjunction with the accompanying drawingswherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded perspective viewof a microwave antenna of the type to which the present invention isparticularly adaptable;

FIG. 2 is a sectional view of the antenna of FIG. 1 taken along the line22 after this portion of the antenna has been filled with the particularmoistureproofing foam of the present invention; :and

FIG. 3 is a further sectional view of the assembled antenna of FIG. 1taken along the line 3-3, showing the regions filled with foam after theprocess of the present invention has been completed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before describing the details ofthe present concept, it should be emphasized that the structuralcharacteristics of the particular antenna to which the moistureproofingmethod herein set forth is to be applied form no part of the invention,and that the novel aspects of applicants process are equally adaptableto all forms of microwave radiators so constructed as to possess one ormore hollow energizable components, or so designed that the electricalconnection to a transmission line is open to the entry of moisture undersevere or extended exposure to the weather. The particular antennachosen for illustration is purely exemplary of such a class ofradiators, and hence an extended discussion of its constructionaldetails will be omitted as unnecessary to an understanding of the uniquefeatures of applicants moistureproofing process.

The microwave device of the drawings represents a so-called agaveantenna of the cubic type that is, it is made up of four similarradiating units 10, 12, 14 and 16 arranged in quadrature. The activeelement of each such unit is a hollow sector or cavity (such as 18 inFIG. 1) from which projects a rod 20 the terminal portion 21 of which isthreaded as shown. This rod 20 extends through a hollow cylindricalmember 22 to which is attached a pair of brackets or straps 24 designedto support the active element 18 in place after the latter is welded orbolted thereto. An insulating bushing 26 is interposed between themembers 18 and 22 when the unit is in assembled position.

The four units 10, 12, 14 and 16 lie generally in a single plane whichin FIG. 2 is that of the paper. Extending normal to this plane andthrough the central axis of the device is a so-called driver assembly,the principal component of which is a hollow tube 28 (FIG. 1) one end ofwhich is flanged at 30 and the other end of which supports a microwavedirector (generally identified by the reference numeral 32) the purposeof which is to create a particular directivity pattern for the antennain a manner well known in the art. The unit 32, both as to itsconstruction and electrical characteristics, has no bearing on theinvention as herein set forth, and is mentioned merely for the sake ofcompleteness. A plurality of bolts 34 secure the director 32 to thetubular member 28 when the antenna is in assembled condition.

A hollow hub 36 encircles the tubular member 28 and is formed with fourradially-extending arms 38, 40, 42 and 44. These arms respectivelysupport and position the four hollow cylindrical members (such as 22)through which respectively extend the four threaded rods (such as 20) toeach of which an active radiating member is attached. It is to be notedthat the outer diameter of each threaded rod 20 is less than the innerdiameter of the radially-extending arm through which it passes, therebyforming a plurality of cavities 46, 48, 50 and 52 which are annular incross-section and which are particularly susceptible to the entry ofmoisture after the antenna has been assembled.

Also shown in FIG. 1 is a so-called Balun unit generally identified bythe reference numeral 54. It consists essentially of a plurality ofelectrical conductors secured to a flange 56 designed to about theflange 30 when the antenna is assembled. Receivable in the tubularmember 28 are four conductors 58, 60, 62 and 64 the length of which ischosen so that the extremities thereof lie within the hollow hub 36 whenassembly of the antenna is complete. The ends of the conductors arethreaded and bent at an angle of 90, so that each conductor lies in aposition to receive one of the rods 20 when the latter is inserted intoits associated hollow cylindrical member 22. The rod is then screwedinto the proper one of conductors 58-64 to establish a tight electricalconnection therebetween.

Two of the conductors 58-64 extend through the flange 56 to becomeassociated with a transmission line (not shown) leading to thetransmitter/receiver with which the antenna is associated. The remainingtwo of the conductors are grounded to the flange, the dimensions of theBalun unit being chosen so that the flange is located at a point wherethe current and voltage relationships permit such action to be taken.Reference to FIG. 3 of the drawings shows the two flanges 30 and 56secured in face-to-face position by the bolts 66.

Since the conductors 58-64 of the Balun unit 54 are physically spacedfrom one another in order to preclude electrical interactiontherebetween, a number of gaps or spaces are formed within the tubularmember 28 between the inner wall of the latter and the outer surfaces ofthe respective conductors. This portion of the antenna, like that in thevicinity of the rods 20, is particularly susceptible to the entry ofmoisture due to breathing with changes in atmospheric pressure.

The device described above is an example of an existing multi-polan'zedUHF antenna of a type now in extensive use. It was originally designedfor optimum operating efficiency in other words, the electricalcomponents of the antenna were matched to the dielectric materialsemployed. In the case selected for illustration, the dielectricsubstance used (the filler material) was air. If this low dielectricmaterial is changed to one of appreciably higher value, it is necessaryto modify the diameter of the Balun conductors 58-64 or to add acompensating transformer as discussed above. Attempts heretofore made tomoistureproof the antenna by adding an ordinary filler material withoutconsidering the factors involved invariably resulted in a severeimpedance mis-match and a consequent drop in operating efficiency. Thepresent concept recognizes that certain conditions must be met, and setsforth specific procedures by means of which the desired results can beobtained.

If the air which originally filled the open spaces within the antenna isto be replaced by a moistureproofing material, then the electricalcharacteristics of such added material must closely approximate those ofthe air which it replaces. Furthermore, the moistureproofing materialmust be capable of completely filling all those regions where entry ofmoisture has been shown to be undesirable, and to be of such a nature asnot to add appreciably to the overall weight of the antenna.

One substance which has been found to be especially suitable for thepurpose consists of a freon-blown foamin-place polyurethane plastic ofthe closed-cell type, having a relatively low density. The electricalproperties of such a substance may be selected so that the specificdielectric constant of the material closely approximates that of the airwhich it replaces. In the case under consideration, the relativepermittivity of the plastic filler, when the latter is formulated in amanner to be later set forth, is 1.058 at 9.375 KMC, with a loss tangentof 0.00048. Thus impedance mis-match is avoided, and operatingefficiency maintained.

One substance especially suitable for the purpose of the presentinvention is made up of the following components in the proportions setforth:

Component A P.B.W. Propoxylated sorbitol (Atlas G2408) 100.0 Toluenediisocyanate, /20 ('I'DI) 390.0

Component B Propoxylated sorbitol (Atlas 62408) 100.0 Tn'ethylenediamine(DABCO) 0.8

Silicone oil (Union Carbide L5320) 1.0 Phosphoric acid, 85% 0.4Fluorocarbon 11 B (Freon) 35 Ratio: 100 parts Component A: 70 partsComponent B The above-described formulation yields a halocarbon-blown,polyether/T DI, rigid urethane foam'with a nominal density of two poundsper cubic foot and dielectric constant of 1.058.

An alternative substance which may be employed if desired in place ofthe one above described is that manufactured by the Olin ChemicalCorporation of New York City under the designation Polysystem 402-0771.It is made up of a polyhydroxyl blend component and a prepolymer (polyoland tolylenediisocyanate) component which, when mixed at a 1:1 pbw(parts by weight) ratio, produce a foam of approxi mately 2 lbs/ft freerise density. The polyhydroxyl resins used in both components arepolyether base type materials, with the polyol component containingtrichlorofluoromethane with minor amounts of silicone and an aminecatalyst.

In preparing the antenna of the drawings for moistureproofing orwaterproofing in accordance with the present invention, the antenna, aswell as the resin and its catalyst, should be at an ambient temperatureof approximately 85 F. A pressure gun is used for injecting thesubstance into the antenna cavities, and, if this gun is designed toaccommodate an 8-oz. polyethelene cartridge, then a number of cups (sayfive, for example) are filled with 120 grams each of Component A, and asimilar number of cups are filled with 84 grams each of Component B (a10:7 ratio). This amount (when mixed) has been found to be sufficient tofill all the cavities of a single antenna except those of the radiatingdipoles per se. These elements are filled separately, as describedhereinafter.

One cup of Component B is then poured into a cup of Component A (after aretarding agent has been added to the latter, as hereinafter described)and the substance thoroughly mixed. The gun cartridge is then filledwith this mixture.

Before proceeding further with a description of the waterproofingprocess, it should be mentioned that the antenna of the drawings isprovided with an injection hole 68 (FIGS. 1 and 3) and four bleed-outopenings 70 respectively formed in the insulating bushings 26. Theinjection hole 68 should be dimensioned to accommodate the nozzle or tip71 of the pressure gun 72 containing the foam cartridge.

The contents of the cartridge are injected into the opening 68 as shownin FIG. 3. It has been found in practice that five cartridges of foamare required to complete the operation, so it is preferable that twooperators should work together, one mixing the ingredients and the otherloading the cartridges and injecting the foam into the antenna. Theprocess should be coordinated to take a minimum amount of time, so thatprevious injections of foam do not harden and block the passage beforeall of the foam has been inserted. After five injections of foam aremade, the injection hole 68 is closed off. The foam in the antennacavities then expands and forces some material out of each of thebleeder" openings 70 in each arm. As soon as this occurs, each opening70 should be blocked off for a short period of time (about seconds) toallow the expanding foam to back fill in the cavities and hence reachall regions to be moistureproofed. After the expiration of thisl5-second period, remove all restrictions from the openings 68 and 70and allow all excess foam to ooze out. After the foam has cured forabout 12 hours, the excess is cleaned away from the openings.

Each dipole or radiating element 18 is filled with foam separately andindependently of the operation described above. For such purpose, eachsuch element 18 is provided with a pair of openings 74 and 76 located onopposite sides of the front edges (FIGS. 1 and 2). After a mixture hasbeen prepared as described above, one cartridge of foam is injected intoeach element 18, the contents of the cartridge being evenly dividedbetween the openings 74 and 76. Following each injection, the remaininghole should be blocked for a short period of time to permit expansion ofthe foam within the cavity and ensure a wetting of all interiorsurfaces.

FIGS. 2 and 3 of the drawings illustrate by stippling those regions ofthe antenna which are filled with foam following completion of theprocess set forth above. It will be noted that all of the air whichformerly occupied the spaces between the electrical components has beenreplaced with foam and, since the latter is impervious to moisture, noshorting or other operating malfunction is likely to occur even undersevere environmental conditions.

The material above described additionally contains a retarding agentwhich allows sufficient time to mix the components, insert the mixtureinto a pressure gun, and carry out the injection process. The retardingagent also maintains the plastic mixture in such form that it is capableof flowing around the internal elements of the antenna to wet all suchsurfaces before hardening occurs. Since the internal passageways of theantenna may be long and restricted in diameter, absence of a retardingagent may result in the foam hardening at or near the entry point beforethe cavity is completely filled. Presence of the retarding agent thusensures sealing of the assembly at the critical points those placeswhere various components are joined together, and especially in theBalun feed unit. This aspect of the invention process is aided by thehigh expansion rate of the foam some 30 to 1 in practice. One suitableretarding agent has beenfound to be benzoyl chloride, C l-l COCl, havinga molecular weight of 140.57. This is mixed with Component A in a ratioof 1 part of retardant to 1,000 parts of Component A. It is important tonote that unless the retarding agent is used, the injected foam willharden before all cavities are completely filled, and operatingefficiency of the antenna will be lowered.

The fumes given off when the foam ingredients are mixed in the mannerabove described are toxic, and exposure thereto should be avoided.Chemical respirators are suggested for the operators, together withadequate ventilation in the work area. Plastic gloves should be worn andclothing protected against spillage. After the foam is cured, however,it can be handled in a normal manner.

It is emphasized that the chemical constituents of the plastic substanceutilized in practicing the present invention are not critical, and thatany material which exhibits the required physical and electricalcharacteristics may be employed. The essence of the concept hereindisclosed is the matching of the electrical impedance of the plasticwhich fills the antenna to the electrical impedance of the member intowhich it is injected. In fact, it has been found that when therespective electrical impedances of the plastic foam and the antennainto which it is injected are mis-matched by a ratio in excess of l to1.1, the beneficial results obtained by practicing the disclosed methodwill not be obtained.

Although plastic waterproofing substances for microwave antennas aredisclosed in the prior art, such disclosures completely lack anyteaching that the dielectric constant of the plastic should approachzero. Hence antennas moistureproofed by such methods can not achieve theoperating efficiency attainable by practicing the process herein setforth.

It should be recognized that applicants process calls for the step ofinjecting into a cavity a plastic possessing a relatively high expansionrate, so that the substance so injected will expand within said cavityto flow over the entire inner surface of the cavity and wet the same.This expansion feature of the plastic is extremely important to insurethat the entire cavity is filled, and hence the plastic is injectedunder pressure. A pour-in-place foam, as suggested in the prior art,does not necessarily possess any expansion characteristics. Such a foamwould not serve applicants purpose regardless of its electricalproperties. This expansion characteristic of the particular foam setforth herein insures that all of the air within the antenna is replacedwith foam. Pouring prior art foam into an open container will notnecessarily achieve this result, as no expansion is ordinarily required.

It is well known to all workers in the electrical art that the closer tounity impedance matching becomes, the more efficient is the operation ofthe device. Any reduction in mis-match is therefore highly desirable. Itis a question of maximum power transfer, and a mismatch of as little as1 to 1.24 significantly reduces the amount of energy that is availablefor utilization.

The substitution of polystyrene for the urethane material abovedescribed is not feasible, since the microwave antenna of the inventionis intended to operate under adverse environmental conditions. Thisembraces all climatic variations, including bright sunlight. Inpractice, antennas exposed to the sun are heated by the impact of thesuns rays on the outer metal surfaces, raising the inner temperature torelatively high levels when such exposure continues for any prolongedperiod of time. Antenna heating also occurs due to the radiation ofhigh-frequency energy from the active surfaces of the assembly, as wellas from a terminal impedance mis-match of the type which the presentinvention is specifically designed to avoid. Antenna heating alsoresults from discontinuities in the current path which affect thestanding wave ratio critical points in the assembly. Reflections fromnearby objects can also produce appreciable heating, and this conditionis especially severe on board naval vessels in an environment where theantenna of the present invention has found widespread use. Many antennaconstructions of the type herein discussed have actually burned out as aresult of this heating. Although the exact temperature reached varieswith antenna design and input power levels, figures of above 200 F arenot uncommon in practice.

The polyurethane foam described above adequately resists these hightemperatures without adverse effects. However, polystyrene foam willnot. A brochure published by the Dow Chemical Company clearly statesthat Because polystyrene is a thermoplastic material,

styrofoam cannot be used where temperatures will exceed 175 F.Furthermore, applicant has no knowledge of any foamin-place polystyrenewhich is available to the public at the present time. Polystyrene isconventionally sup-' plied in the form of beads which are molded in thepresence of heat. How such a material can be employed in the processspecifically described by applicant'is by no means clear, nor is itunderstood how such chemical can be injected under pressure into anantenna, following which it will foam in place and expand to wet theentire inner surface of the assembly.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

I claim:

1. A multipolarized microwave antenna of the type formed as a complexnetwork of hollow radiating elements of elongated configuration andrestricted diameter which is subject to operational temperatures inexcess of 175 F and having a plurality of cavities containingenergizable components electrically isolated from one another,comprising a. a dielectric material which precludes entry of moistureinto said antenna and electrically isolates the energizable componentsfrom one another, completely filling each cavity of said antenna;

b. said dielectric material consisting of a foamed-inplace low-densitypolyurethane plastic substance having specific dielectric constantapproximately equal to that of air but in no event exceeding 1.1, saidplastic substance including a retardant and the following components inthe ratio set forth:

Component A P.B.W. Propoxylated sorbitol 100.0 Toluene diisocynate, /20390.0

Component B Propoxylated sorbitol 100.0 Triethylenediamine 0.8 Siliconeoil 1.0 Phosphoric acid, 0.4 Flurocarbon it B 35 Ratio: parts ComponentA to 70 parts Component B, said plastic substance having a dielectricconstant of approximately 1.058, said retardant is benzoyl chloride in aratio of one part thereof to 1000 parts of Component A; and

